This invention relates to a supercritical conditions, gas phase process for catalytically isomerizing a xylenecontaining stream to a product rich in p-xylene (pX) and thereafter reducing the temperature of said product such that it passes directly from said gas phase to a liquid phase without a phase change, and, more particularly, to a supercritical conditions, gas phase process in which both temperature and pressure are above their critical values, to isomerize a xylene-containing stream, optionally containing a minor amount of ethylbenzene (EB), to a product rich in p-xylene over a catalyst, and thereafter reducing the temperature of the product such that it passes directly from said gas phase to the liquid phase without a phase change.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by isomerization followed by, for example, lower-temperature crystallization of the p-xylene with recycle of the crystallizer liquid phase to the isomerizer. Principal raw materials are catalytically reformed naphthas and petroleum distillates. The fractions from these sources that contain the C.sub.8 aromatics vary quite widely in composition but will usually contain 10 to 35 weight percent ethylbenzene and up to about 10 weight percent primarily C.sub.9 paraffins and naphthenes with the remainder being primarily xylenes divided approximately 50 weight percent meta, and 25 percent each of the ortho and para isomers. Feeds that do not have the primarily C.sub.9 paraffins and naphthenes removed by extraction are termed "unextracted" xylene feeds.
The xylene isomerization process is an important step in the eventual production of polyester based upon terephthalic acid. The p-xylene isomerization product is oxidized to terephthalic acid, conveniently by a cobalt ion/acetic acid, liquid-phase oxidation, and serves as a raw material for the production of polyethylene terephthalate.
Xylene isomerization is performed commercially in the vapor phase or the gas phase at pressures well below the critical pressure of the xylene stream using a catalyst that exhibits catalytic activity for both the isomerization of xylenes and the conversion of at least a portion of the ethylbenzene impurity present in the feedstock. Isomerization in the liquid phase is preferred, but a number of problems, including low catalyst activity at temperatures required to maintain reactor contents in the liquid phase, are encountered. In a supercritical conditions xylene isomerization process, higher temperatures would be used so the problem of low catalyst activity associated with liquid phase operation is reduced.
It is common practice to add substantial amounts of hydrogen during vapor phase or gas phase xylene isomerization, as currently practiced, in order to slow catalyst deactivation. Hydrogen-to-hydrocarbon mol ratios are typically in the range from about 2 to about 10. The operation of a recycle gas compressor to supply such amounts of hydrogen can represent a substantial expense, and elimination of the recycle gas compressor has been cited as an incentive for liquid phase isomerization. It has been discovered that by proper choice of catalyst, acceptable catalyst life can be obtained during xylene isomerization at supercritical conditions without the addition of hydrogen and the recycle gas compressor eliminated in supercritical conditions isomerization as well.
Compared to current processes, a supercritical conditions xylene isomerization process also can offer substantial reduction in fuel for the furnace that is used to increase the temperature of the reactor feed to the desired reactor temperature. This reduction is due to higher average heat transfer coefficients for supercritical fluids.
In current processes a temperature pinch point occurs in the feed-effluent heat exchangers used to preheat the reactor feed and cool the reactor effluent at a point internal to the heat exchanger, i.e., not at either the hot or cold end. Temperature approach is defined as the difference in temperature between the heated and cooled streams at a point within the heat exchanger. A temperature pinch point is defined as a point where there is a minimum in temperature approach. The minimum in currently used processes can be substantially below the temperature difference between the hot and cold streams at other points within the heat exchanger, and in particular, substantially below the temperature difference at the hot end of the heat exchanger.
Since the rate of heat transfer is generally proportional to temperature approach, it is desirable to maximize temperature approach at points within the heat exchanger. However, the temperature approach at the hot end of the heat exchanger, where the reactor effluent enters the exchanger and the feed stream exists, is essentially proportional to fuel consumption in the furnace ahead of the reactor. Thus, it is desirable to minimize temperature approach at the hot end of the feed-effluent heat exchanger. The low average heat transfer coefficients and the temperature pinch point limit the temperature approach at the hot end of the feed-effluent heat exchanger that can be achieved economically for existing vapor or subcritical gas phase processes. The temperature approach at the hot end of the feed-effluent heat exchanger is in the neighborhood of 80.degree.-120.degree. F. for currently practiced processes. This means that for current isomerization processes the temperature of the feed stream to the reactor must be increased by an additional 80.degree.-120.degree. F. by passing the stream through a furnace, for example, after it leaves the feed-effluent heat exchanger. The heat required to effect the temperature increase represents a cost associated with the process. The desire to lower temperature approach at the hot end of the feed-effluent heat exchanger is balanced by the fact that lowering the temperature approach at the hot end of the feed-effluent heat exchanger requires increasing the surface area of the exchanger, which also increases capital costs. By operating above the critical pressure, it is possible to substantially reduce the magnitude of the temperature pinch or to move the point of lowest temperature approach to the hot end of the feed-effluent heat exchanger. Together with the higher average heat transfer coefficients, this will allow a lower temperature approach at the hot end of the feed-effluent heat exchanger for given heat exchanger surface area, and therefore, reduce fuel consumption.
Isomerization of xylenes in the liquid phase has been a subject of study by a number of workers. See for example, U.S. Pat. Nos. 3,777,400; 3,856,871; 4,268,420; and 4,269,813; Japanese Kokai 57-32233 (1982); and a 1972 article in "Hydrocarbon Processing" at p. 85 by P. Grandio, F. H. Schneider, A. B. Schwartz and J. J. Wise. In these reports, the primary reaction observed was isomerization of xylenes, even in the presence of ethylbenzene and other impurities. Most of the catalysts which were employed in the above works contained zeolites of the large-pore type, e.g., faujasite-type zeolites or mordenite.
U.S. Pat. No. 3,856,879 is an early report of the use of shape-selective, molecular-sieve-containing catalysts for the isomerization of xylenes and the conversion of ethylbenzene in the liquid phase. The aluminosilicate zeolites ZSM-5, ZSM-12, and ZSM-21 are recommended for use in this process. Although by-product distributions are not reported in the patent, the xylene feed is said to be isomerized to its equilibrium isomer concentrations and the ethylbenzene converted via the transalkylation and disproportionation mechanisms. A catalyst containing the aluminosilicate molecular sieve, ZSM-5, is claimed to exhibit no deactivation, even in the absence of hydrogen.
U.S. Ser. No. 285,105, filed Dec. 15, 1988, now U.S. Pat. No. 4,962,258, teaches that gallium-containing molecular sieve catalyst compositions are effective for liquid phase xylene isomerization as their deactivation rate is less than that of similar catalyst compositions which have been found effective for vapor or gas phase isomerization.
While commercial chemical reactions have not been carried out under supercritical conditions, there has been commercial interest in the use of extraction and separation processes above the critical temperature. This interest is based upon the greatly increased solubility and improved mass transfer rates obtained when solvents are at elevated temperatures and pressures. Much less has been written on the effect of supercritical pressures and temperatures on reactions, particularly heterogeneous, catalyzed reactions.
At pressures above the critical pressure of the reactant mixture, it is possible to isobarically heat or cool the reactor feed and effluent and pass back and forth between the liquid and gas phases without phase separation. This factor would allow the design of a more efficient feed-effluent heat exchanger and allow a reduction in fuel consumption in processes which require a furnace in front of the reactor.
In the case of xylene isomerization, another benefit can be realized if the use of supercritical conditions allows the elimination of hydrogen (or at least its substantial reduction) which is normally a component of the feed to a currently practiced vapor or gas phase xylene isomerization. The reduction or elimination of hydrogen allows size reduction or elimination of the recycle gas compressor which reduces energy consumption. A third advantage to be found in xylene isomerization under supercritical conditions relative to liquid phase processes is the possibility that catalyst activity will be increased at the higher temperatures required to achieve supercriticality and so smaller reactors and smaller catalyst loads would be required compared to liquid phase processes. Opposed to these benefits, the higher pressures required in supercritical operations require that thicker-walled reactors and other equipment such as heat exchangers are needed. Also, the higher temperatures and pressures employed in a catalyzed reaction such as xylene isomerization put more demand on catalyst properties as the usual isomerization catalysts deactivate more rapidly at higher temperature.
Now it has been found that these and other benefits can be obtained by carrying out the xylene isomerization under supercritical conditions employing selected isomerization catalysts and operating in a region of both supercritical temperature and supercritical pressure.